15 Dec Winter 2016-2017

Winter is always a very interesting season to forecast.There are numerous items to analyze and missing any of those key items could be disastrous to the accuracy of a forecast.In this newsletter, before presenting the forecast, I will outline what I think will be the most likely influences of 2016/17 Winter weather…Arctic Ice, La Nina, and Soil Moisture.

Arctic IceThis summer, Arctic ice reached its second-lowest areal extent in the satellite record of 35 years.Oddly enough, Arctic summer-time air temperatures were a bit cooler than average and air pressures were generally lower than average due to increased Arctic storms this summer.As a result, a few months ago it was surmised that extreme sea ice loss would not occur this year as those two factors point to reduced ice loss.

But, ice loss occurred in a rather big way.The minimum this year was reached on September 10.Ice scientists believe that the much lower than normal coverage was exacerbated by two tropical cyclones in the Pacific Ocean which greatly increased warm air advection, wave activity, and oceanic thermal mixing during late August and the first week of September. Recall that ocean waves can travel long beyond their origination.These are called oceanic swell waves.With the increased flow of warmer-than-normal air from the cyclones and the mixing caused by wave activity, ice loss was rather drastic.

However, sea ice will continue to build (as it normally does beginning in late summer/early fall) as the polar low expands in areal extent and intensifies (figure 1, courtesy of the National Snow and Ice Data Center).It is the building of sea ice which leads to a steady supply of cold air for transport into the mid-latitudes of North America via meridional kinks in the jet stream.Currently, areal coverage of sea ice (in square kilometers) is at record-low levels (figure 2, courtesy of the National Snow and Ice Data Center)

when looking at the past 35 years.However, in studying the week-to-week changes in areal extent, the last week or two has seen significant growth, especially into the Northwest Passages south into Hudson Bay.The further expansion of Arctic ice warrants watching.

La NinaThis time last year, we were preparing for a major El Nino event.That event happened and happened with some gusto.Beginning in April 2016, there were grumblings about a La Nina event for this Winter.At one point this summer, probabilities were listed at 70% that a consistent La Nina would occur.As of 3 months ago, although we had seen some eastern tropical Pacific Ocean sea surface temperature cooling (as would be the case with a La Nina event), it had not proceeded to the extent most scientists believed it would have by that point in time.Therefore, the “La Nina Watch” conditions were dropped in favor of “neutral”.For the record, “neutral” conditions are fairly typical of a “normal” Fall/Winter set-up.

As scientists are not seeing a steady and pronounced cooling in the eastern tropical Pacific Ocean, probabilities now favor a “weak La Nina” status continuing through January 2017, as opposed to a definitive, steady-state La Nina event. In October, the official outlook was to have a weak Nina status develop by December (which happened!) only to relax into the “neutral” category by February (remains to be seen).That is quite the decrease from a 70% La Nina environment which was forecasted as the consensus this past summer.

At the most, if a consistent La Nina were to happen, it would be a weak Nina.As for its impact on this winter, neutral and weak Nina are not very different with respect to climate impacts as La Nina is just an amplification of the “normal” winter set-up in the tropical Pacific Ocean.It is El Nino which is a marked change in the environment from “normal” and has sometimes devastating weather impacts from droughts to floods in various parts of the world.A strong Nina would have more of a climatic impact as well.

Soil Moisture

Soil moisture is important during winter in a similar vein that it is important during summer….the higher the moisture, the less extreme the air temperatures will be. Currently, there are 3 areas of moderate/severe drought: central CA, the Southeast, and New England (figure 3, courtesy of the National Drought Mitigation Center). Without significant rainfall in these areas, any cold air advection will be amplified.

Winter Outlook

What does all of this mean in terms of climate impacts????All signs are pointing to a generally cooler-than-normal pattern for the northern tier east of the Rockies for the November-March period (figure 4).The cold air pool in Canada should be “healthy enough” and because of a lack of mechanisms (due to a weak Nina) to keep the jet stream bottled-up in Canada, southerly intrusions of Arctic air will occur, even later in the winter season.The coldest area will be in the Northeast (especially given the dry conditions) with decreasing cold anomalies (but still overall cool anomalies) into the Midwest and Plains.Look for an active lake-effect snow season as the more opportunity for frontal passages will allow for wind with a northerly component to blow across the Great Lakes.For the most part, the Southwest through Texas will be warm and the Northwest and Inter-Mountain West will both be “normal”.

Keep in mind that these forecasted anomalies are for the aggregated November through March time frame.There will be weeks of back-and-forth cooler/warmer/normal weather no matter where in the country you reference.

As for risks to this winter outlook, it is probably in the placement of the coldest anomalies.If the Arctic air takes a “pathway of entrance” more towards the Plains, that will shift the coldest anomalies west into the Midwest and Plains.This winter hinges on “where” the cold will flow into the country, and less on “if” the cold flows into the country.